1. Field of the Invention
The present invention relates generally to transceiver front-end circuits and more specifically to a highly integrated transceiver front-end circuitry that couples a single antenna port to both a balanced receiver input and a balanced transmitter output.
2. Description of the Related Art
Highly integrated radios typically have differential inputs on the receiver and differential outputs on the transmitter. The differential inputs on the receiver allow the receiver to reject common mode signals (e.g., noise) and variations. By way of example, a differential input can detect input differential signals that a single ended (i.e., a non-differential) receiver might not be able to discriminate from a noise in the common mode. Similarly, a differential output on the transmitter can more efficiently transmit the power (e.g., biasing current) that is applied to the output power amplifier stages of the transmitter.
FIG. 1 shows a prior art, time-division duplex front-end circuit 100 for coupling a differential receiver 114 and a differential transmitter 138 to a single antenna port 101. The receiver 114 and the transmitter 138 each have different requirements for optimum performance. The input to the receiver 114 must be optimized impedance for both the best conjugate match and the best noise performance. The receiver impedance matching is typically provided by a receiver matching network 108.
Ideally, the antenna 102 is matched to the receiver 114 to produce the optimum small signal transfer by tuning the receiver inputs 112p, 112n and the antenna 102 to achieve a conjugate match. If the antenna 102 and the receiver inputs 112p, 112n do not have a conjugate match, then the performance of the receiver 114 can be degraded. By way of example, the gain of the receiver 114 may be reduced and the noise produced by the receiver can also be increased if the antenna 102 is not properly matched to the receiver inputs 112p, 112n. For optimum receiver performance, the receiver input stages (i.e., the low noise amplifier (LNA) 112) should be designed so that the conjugate match and best noise match are at the same impedance. If the conjugate match and best noise match are at the same impedance then the receiver matching network 108 can be specifically designed to the required impedance.
The output of the transmitter 138 must also be impedance matched to the antenna port 101 to provide the most efficient conversion of the bias current applied to the transmitter power amplifier 136 to output transmit power. Typically, a transmitter matching network 134 matches the impedance of the output of the transmitter 138. Unfortunately, the receiver 114 input impedance and the transmitter 138 output impedance are typically not the same and therefore separate matching networks 108, 134, respectively, are required.
It is also desirable to disconnect the receiver 114 input from the antenna 102 when the transmitter is transmitting so that the transmit power is not applied directly to the input of the receiver. Applying the full transmit output power to the input amplifiers (e.g., low noise amplifier (LNA) 112) of the receiver 114 can damage the LNA 112 or at the very least reduce the overall receiver sensitivity (e.g., ability to discriminate between noise and signal) by introducing large amplitude harmonic noise to the receiver 114. Further, if the receiver 114 is not disconnected from the antenna during transmit mode, the amount of transmitter power transferred to the antenna can be reduced due to loading of the receiver 114 on the transmitter 138. If the transmitter 138 is coupled to the antenna 102 during the receive mode, the transmitter 138 can also “load” the receiver circuit which can reduce the net input signal to the receiver 114. For the above reasons, a transmit/receive switch (T/R switch) 104 is typically included to switch the antenna 102 between the receiver 114 and the transmitter 138.
A balun is a circuit that includes a transmission line transformer for converting balanced input to unbalanced output or vice versa. A balun may or may not provide wide frequency range impedance transformation depending upon the configuration used. The receiver balun 106 converts the unbalanced, single pole antenna 102 to a differential signal for the receiver 114. The transmitter balun 130 converts the balanced transmit signal to an unbalanced signal for the unbalanced, single pole antenna 102. The receiver balun circuit 106 and the transmitter balun circuit 130 are typically transformers or other types of inductive devices that can match the impedance of the unbalanced antenna port 101 to the balanced impedance of the receiver 114 and the transmitter 138, respectively.
Referring again to FIG. 1, the receive path begins with the receive signal entering the antenna 102. The antenna 102 is coupled to the single antenna port 101 that is coupled to the transmit/receive (TR) switch 104 at the antenna terminal 104A. The TR switch 104 has two terminals 104B, 104C opposite the antenna terminal 104A. The input 106A of a receiver balun 106 is coupled to terminal 104B. The differential outputs 106n, 106p of the receiver balun 106 are coupled to the corresponding differential inputs of the receiver matching network 108. The differential outputs 108n, 108p of the receiver matching network 108 are coupled to corresponding differential inputs 112n, 112p of the receiver LNA 112. The receiver LNA 112 represents the first stage or stages of the receiver portion 114 of the transceiver 120.
In the transmission signal path the differential transmitter signal is output from the transmitter PA 136 outputs 136n, 136p. The transmitter PA 136 represents the final output stage or stages of the transmitter portion 138 of the transceiver 120. The differential transmitter outputs 136n, 136p are coupled to the corresponding differential inputs 134n, 134p of the transmitter matching network 132. The differential outputs of the transmitter matching network 132 are coupled to the corresponding differential inputs 130n, 130p of the transmitter balun 130. The unbalance output 130A of a transmitter balun 130 is coupled to terminal 104C of the TR switch 104.
The TR switch 104 provides the ability to connect only one signal path (transmit or receive) at a time to the antenna so that the transmitter 138 and the receiver 114 can share a common antenna. For example, in the receive mode, as shown, the TR switch 104 couples signals coming in the antenna 102 to the input 106A of a receiver balun 106. Also in the receive mode, transmit signal path (i.e. from the PA 136, through the transmitter matching network 132 and through the transmitter balun 130) is not coupled to the antenna 102. In this way the TR switch 104 prevents the transmitter 138 from transmitting into the receiver 114 or from reducing (loading down) the net receive signal input to the receiver 114.
Conversely, in transmit mode, the TR switch 104 is in the transmit position (not shown) and the transmit signals can be coupled from the PA 136, through the transmitter matching network 132, through the transmitter balun 130, and across terminal 104C to terminal 104A of the TR switch 104 and out the antenna 102.
In conventional, highly integrated radio, the TR switch 104 is located off the package that contains the front-end circuit 100. The TR switch is located off the package because of size limitations and because the TR switch is an active component (e.g., transistor, diode or integrated circuit). Further, the typical TR switch 104 can be constructed from PIN diodes and can be very inefficient because a solid state TR switch 104 requires a high current to produce a low resistance signal path in at least one of the receive signal path or the transmit signal path. High current use is not optimum for mobile applications such as cellular telephones or Bluetooth applications. Further, because the TR switch is located off package, then the parasitic components of the connections between the TR switch 104 and the package can vary from design to design. This variation requires either that the matching networks 108, 132 must be detuned to compensate for the range of the variation thereby resulting in a less than optimum match with the receiver 114 and the transmitter 138. Alternatively, the matching networks 108, 132 will typically require specific tuning to compensate for the manufacturing variation. Either option is less than optimum as the resulting front end circuit 100 will not be fully optimized and/or will require individual application optimization.
Another conventional transceiver front-end circuit includes a single balun that is used by both the receiver and transmitter. Another conventional transceiver front-end circuit includes a PIN diode circuit in series and shunt combined with an external balun. PIN diodes require significant currents that therefore reduce power efficiency and are therefore less than optimal for mobile/portable applications. A bias current of several mA may be required to achieve low resistance current flow across a PIN diode.
Conventional transceiver front-end circuits typically use substantially complicated PIN switch configurations. One such configuration combines parallel connections of the PA and LNA connection and an external balun. Another conventional transceiver front-end circuit connects the PA outputs and the LNA inputs, with parasitic absorption, and an external balun. In this version, the PA acts as a capacitance when the PA is turned off so that the LNA port can effectively receive.
Yet another conventional transceiver front-end circuit has separate paths, which simplified the application that required an external LNA topology. This approach also requires on-board tuning because exact tuning will be dependent on the application circuit, the dielectric materials variation, component and assembly variations, etc. On-board tuning also increases time-to-market in many applications.
In view of the foregoing, there is a need for a front end circuit that provides efficient switching between transmit mode and receive mode, without an off package TR switch and provides optimized matching for both the receiver inputs and the transmitter outputs.